The sea urchin appears to have an immune response that is similar to what is found in the vertebrates. Autografts are accepted, all allografts from unrelated individuals of the same species are rejected. Some allografts from related individuals are accepted; the degree of acceptance being proportional to the level of genetic relatedness. Second set allografts are rejected much faster than first set grafts. Bacteriophage also act as antigen. There is evidence of a graft vs. host response. Overall, we plan to investigate the nature of this immune response using several approaches. The genetic basis will be studied with the use of our colony of inbred sea urchins. With these we should be able to determine the number of histocompatibility loci. We will also be determining what cells and organs mediate the reaction to foreign substances. Current evidence implicates the coelomocytes. The time in development when immunocompetence arises and whether or not individuals can be made tolerant to foreign substances will be determined with grafts and injection of foreign substances into animals at various stages in the life cycle. We will also be using chimeric animals that can be formed by fusing blastomeres. The sea urchin's response to viruses and proteins will be studied in detail. Additionally, we will examine immunosuppressson using various protein synthesis inhibitors and other compounds that commonly are used on vertebrates.